1. Field of the Invention
The present invention generally relates to receivers and, more particularly, is directed to a receiver for eliminating a crosstalk of an adjacent broadcast which is suitable to be applied to a television receiver used in the United States of America (U.S.A.).
2. Description of the Prior Art
FIG. 1A of the accompanying drawings shows a schematic diagram illustrating a frequency arrangement or spectrum of radio frequency (RF) signals used in the United States.
Referring to FIG. 1A, there are shown RF signals of U.S.A.-6 channel (US-6ch) of a television broadcast and RF signals of FM channels (FM-201 channel (FM-201ch) to FM-220 channel (FM-220ch)) of frequency modulation (FM) broadcasts. The FM broadcasts are arranged with a frequency distance of 200 kHz unit basis. In FIG. 1A, fp=83.25 MHz represents a video RF frequency and fs=87.75 MHz represents an audio RF frequency.
FIG. 2 of the accompanying drawings shows a block diagram illustrating an example of a conventional tuner 14 for a television receiver for receiving television broadcasts in the United States.
Referring to FIG. 2, an RF signal 2 supplied to an input terminal 1 is then applied to a mixer 8 through a bandpass filter (BPF) 3 for a very high frequency (VHF) entries, an FM trapping circuit 4, an automatic gain controller circuit 5, and an interstage circuit 6, thereby being converted into an intermediate frequency (IF) signal 9. The mixer 8 is connected with a local oscillator 7. The IF signal 9 is amplified by an amplifier 10 and then applied to an output terminal 12 as an amplified IF signal 11. An image IF frequency and an audio IF frequency of each of the IF signals 9 and 11 are 45.75 MHz and 41.25 MHz, respectively.
The FM trapping circuit 4 is provided for preventing the interference or crosstalk in the television signal by the FM broadcast upon receiving the television broadcast. The FM trapping circuit 4 has a FM trapping or selectivity characteristic shown in FIG. 1B, wherein a selectivity curve 15 is set to have a low quality factor (Q) in order to cover all FM channels of the FM-201ch to FM-220ch. Further, the selectivity curve 15 represents a maximum attenuation of about 10 to 15 dB almost at a center frequency band of the FM channels and an attenuation of about 2 to 3 dB at both ends of the FM channels, that is, at a frequency band of the FM-201ch, for example.
However, since the selectivity curve has a low Q, the interference or crosstalk due to the FM channels such as the FM-202ch, FM-203ch etc. having a higher RF frequency than the FM-201ch can be prevented, but the interference or crosstalk due to the FM-201ch adjacent to the RF signal of the television broadcast can not be prevented. Thus, there was a problem that the video and audio signals of the US-6ch having an interfering component appear at a detected output terminal 38 (FIG. 3). The interfering signal included in the IF signals 9 and 11 of the FM-201ch has a frequency fi' of 40.9 MHz.
FIG. 3 of the accompanying drawings shows a block diagram illustrating an example of a conventional television receiver. As shown in FIG. 3, in order to effectively prevent the crosstalk due to the FM-201ch, a trapping circuit 13 for trapping the interfering signal of 40.9 MHz included in the IF signals 9 and 11 of the FM-201ch is provided in an IF circuit 17 arranged between the tuner 14 and a detecting circuit 16. A surface-acoustic-wave (SAW) filter 18 with a wide pass band and an IF amplifier 19 are inserted in the IF circuit 17 between the trapping circuit 13 and the detecting circuit 16.
In the television receiver shown in FIG. 3, however, since the trapping circuit 13 is fixedly arranged in the receiver, the wide pass band of the SAW filter 18 is narrowed due to the subsidiary function, that is, attenuation function of the trapping circuit 13. Thus, the conventional television receiver has the disadvantage that all channels of the television broadcasts including the US-6ch can not be detected.